Complex refractive indices in the ultraviolet and visible spectral region for highly absorbing non-spherical biomass burning aerosol

Abstract. Biomass burning aerosol is a major source of PM2.5, and significantly affects Earth's radiative budget. The magnitude its effect poorly quantified due to uncertainty in the optical properties formed from biomass burning. Using broadband cavity-enhanced spectrometer with recently increased spectral range (360–720 nm) coupled size-selecting inlet, we retrieve complex refractive indices throughout near-ultraviolet visible region. We demonstrate index retrievals for two standard samples: polystyrene latex spheres ammonium sulfate. then 13 controlled fires during 2016 Missoula Fire Science Laboratory Study. that technique highly sensitive accuracy size distribution method find while can constrain brown carbon many fires, fresh smoke dominated by fractal-like black presents unique challenges not well-represented Mie theory. For show theory decreases as fraction mass increases. At 475 nm, average 1.635 (±0.056) +0.06 (±0.12)i, at 365 1.605 (±0.041) +0.038 (±0.074)i.